Modeling water quality in lakes or reservoirs is very difficult, due to uncertainties and complexities in vertical and longitudinal stratification and other processes that prevent complete mixing of the water in storage. In general, if you are modeling water quality in a river (except if using QUAL2K), it is best for you to estimate the water quality in the reservoir and enter as data the concentrations of each constituent in water released from the reservoirs.
However, under certain circumstances (small reservoir, conservative constituent, monthly timestep), you may choose to model the reservoir outflow water quality using one of two functions. Use the drop-down menu in the expression box to choose either Same as Inflow or SimpleMixing. Please use these two functions with caution, as they may give inaccurate results for reservoirs that are not thoroughly mixed.
Same as Inflow means that the outflow water quality concentration in any given timestep will equal the (mixed) concentration of all inflows to the reservoir in that timestep. Note that this is a very large simplification, and does not attempt to model the changing water quality in the reservoir's storage. This would only be suitable if the inflow volume to the reservoir was much larger than the volume in storage.
SimpleMixing will calculate the concentration of water in storage in the reservoir, starting from its initial concentration (the initial condition for the first timestep of the simulation is given as the parameter to the SimpleMixing function, thereafter it is the concentration from the previous timestep), adding in any pollution loads from inflows to the reservoir. Evaporation will increase the concentration, because the water that evaporates carries no pollution. This function is only available for conservative constituents -- it cannot be used for temperature, BOD, DO or any constituent with first-order decay. For reservoirs with SimpleMixing, you can see the results for their concentration in the Results View report: Water Quality, Reservoir Storage Water Quality (Mixed). This report is only available for reservoirs that use SimpleMixing.
Leave blank if not modeling water quality in the river. When using QUAL2K to model water quality in the river, you do not need to enter reservoir outflow water quality. In this case, the outflow water quality will be calculated by QUAL2K.
Entered on: Data View, Branch: Supply and Resources \ River \ <River name> \ Reservoirs \ <reservoir name>, Category: Water Quality, Tabs: <Constituent Name> Concentration